Thermocouples are temperature measuring devices which measure temperature by employing dissimilar metal conductors joined at a point or junction where the temperature is to be measured with free ends connected to an instrument to measure a voltage generated across the junction of the dissimilar metals. The bimetallic junction of dissimilar metals has been formed of various metals which provide a thermoelectric differential between the two metals upon exposure to heat.
Conventional devices use a variety of materials to produce thermocouple sensors with high operating temperatures. The various metals used to form thermocouple sensors suffer from the detrimental effects of contamination, ionic migration, sublimation, oxidation and substantial decrease in mechanical strength with increasing operating temperatures. Current sensors are thus limited to an operating envelope of less than 1090° C. (1994° F.) to ensure long term, stable output with minimum drift in resistance. Higher temperature sensors can operate to temperatures up to 2370° C. (4298° F.) but are either limited to specific environmental conditions (such as for instance: a vacuum environment, an inert gas environment, or a hydrogen atmosphere) and/or must be limited to short term operation to prevent premature failure. This temperature operating range has limited the application of these sensors in hostile, high temperature systems such as those commonly encountered in the aerospace, petroleum and glass industries.
Prior art thermocouple sensors have had the disadvantage of melting at fairly low temperature and have required insulation and various sheathing systems to protect the thermocouple during operation at prolonged elevated temperatures. However, this sometimes results in undesirable reactions between the metals in the thermocouple sensor and the materials used in the insulation and sheathing systems.
The problems of undesirable reactions in thermocouple sensors have been aggravated by the temperatures encountered in nuclear reactor systems, rocketry heat sensors, high-temperature and vacuum processing and other applications where temperature measurements at or above 1500° C. (2730° F.) are involved. Thermocouples have utilized sheathing and insulation in an effort to prevent the disintegration of the thermocouple in such systems. The insulation and sheathing systems have the further disadvantage of resulting in time delays in obtaining temperature readings due to the insulation and mechanical packaging designed implemented to prevent failure resulting from such problems as gas leakage at the thermocouple sheath seals, cracked sheaths and other mechanical limitations imposed by ceramic insulated metal sheathed thermocouple sensors.
Prior art bimetallic bare sensor combinations, such as those formed from tungsten and rhenium have generally not proven to be uniformly reliable or to have a useful operational life at extended temperatures due to breakage of the thermocouple hot junction upon initial heating and drifts in EMF temperature relationships. These problems are believed to be the result of thermal and chemical phase transitions and of preferential evaporation of one of the metals in the bimetallic sensor. These sensors are thus limited to vacuum, inert, or hydrogen atmospheres.
Attempts to extend the operational range of thermocouples have typically been limited to use of insulation and sheathing techniques or increasing the high temperature properties of known materials through alloying processes or coatings, the disadvantages of which have been discussed above.
High melting, noble metal thermocouples made of e.g., platinum (Pt), rhodium (Rh), palladium (Pd), iridium (Ir) and alloys thereof are known in the art. For example, some widely used thermocouples for measurement of temperatures above 1000° C. (1830° F.) are: (Pt/Pt—13% Rh); (Pt/Pt—10% Rh); and (Pt—6% Rh/Pt—30% Rh). Each leg of the thermocouple is made of a wire or thin film of Pt and/or Pt—Rh. The EMF-temperature responses for these devices, the basis of temperature measurement via thermocouples, are moderate and oxidation resistance is good. These thermocouples can be used with moderate to severe drift (i.e., a change in EMF with time due to any cause such as composition change, oxidation or chemical attack) up to 1500° C. (2730° F.). Other noble metal elements, e.g., palladium and iridium, and precious metal elements, e.g. gold and silver or alloys thereof with platinum are also useful to form thermocouples. Such thermocouples, however, are not widely used because they are more susceptible to oxidation than platinum, and degrade by drift caused by selective oxidation.
Some of the characteristics of platinum can be improved by the usual alloy hardening method of adding a metal to the platinum base, followed by heat treatment. However, problems can occur after alloying. For example, when a high concentration of any alloying element is added to the platinum base, the electrical properties of the resulting platinum limb become inferior; at the same time the hardening phase will partially or totally dissolve into the base at high temperatures, thus the effects of the hardening action will be reduced.
Dispersing oxides of transition metals or rare earth metals within noble or precious metals is an example of a method of creating thermocouple materials with the desired extended temperature properties. For instance, dispersion hardened platinum materials (Pt DPH, Pt—10% Rh DPH, Pt—5% Au DPH) are useful materials because they achieve very high stress rupture strengths and thus permit greatly increased application temperatures than the comparable conventional alloys.
Dispersion hardening (DPH) creates a new class of metal materials having resistance to thermal stress and corrosion resistance that is even greater than that of pure platinum and the solid solution hardened platinum alloys. When operational life, high temperature resistance, corrosion resistance and form stability are important, a sensor can be manufactured of DPH platinum and can be used at temperatures close to the melting point of platinum.
Dispersion hardened materials contain finely distributed transition element oxide particles which suppress grain growth and recrystallization even at the highest temperatures and also hinder both the movement of dislocations and sliding at the grain boundaries. The improved high temperature strength and the associated fine grain stability offer considerable advantages.
DPH of platinum has been developed and applied to for instance, the glass industry. For instance, zirconia grain stabilized platinum has been used in the glass industry for the construction of a sheet of material. This approach however, has not previously been used in the measurement field. For instance, the glass industry is focused on stability of the material at high temperature, whereas in the measurement field not only is material stability at high temperature a concern but signal repeatability and quality are critical. In addition, some of the various DPH of platinum approaches taken have utilized a powder material that cannot be utilized and manufactured into a wire for use in a measurement device. Therefore, these techniques are not usable for the measurement field.
Platinum: Platinum-Rhodium Thermocouple Wire: Improved Thermal Stability on Yttrium Addition Platinum, By Baoyuan Wu and Ge Liu, Platinum Metals Rev., 1997, 41, (2), 81-85 is incorporated by reference. The Wu article discloses a process of dispersion hardening platinum for a platinum/platinum-rhodium thermocouple wire which incorporates traces of yttrium in the platinum limb.
As described in the Wu article, the addition of traces of yttrium to platinum as a dispersion phase markedly increases the tensile strength of the platinum at high temperature, prolongs the services life and improves the thermal stability. Yttrium addition prevents the growth in the grain size and helps retain the stable fine grain structure, as the dispersed particles of high melting point resist movements of dislocations and make the materials harder. The strength of a material is related to the movement and number of the dislocations.
In order to harden metals, the movement of the dislocations needs to be restricted either by the production of internal stress or by putting particles in the path of the dislocation. After the melting and annealing process, the majority of the trace yttrium (in the dispersion phase of the platinum) becomes yttrium oxide, which has a much higher melting point than platinum. When the temperature is near the melting point, dispersion hardened particles fix the dislocation, thus hardening the platinum and increasing its strength.
At the same time the grain structure becomes stable after dispersion hardening and there is also microstructural hardening. The dispersed particles affect the recrystallization dynamics, inhibit rearrangement of the dislocations on the grain boundaries and prevent the movement of the grain boundaries. Therefore, this dispersion hardened platinum possesses a stable fine grain structure at high temperature.
The Wu thermocouple meets the output requirements of the Type S standard for thermocouples—those made of Pt: Pt-10% Rh—whose manufacturing tolerances are prescribed by the International Electrotechnical Commission (I.E.C.). Because the platinum-rhodium leg of a conventional thermocouple has much higher tensile strength than a pure platinum leg, the Wu thermocouple dispersion hardened only the platinum leg in order to increase the tensile strength of the platinum leg to balance the strength of the two legs. The Wu thermocouple did not use dispersion hardening in both legs and did not face the challenge of obtaining a repeatable output signal from the thermocouple at an extended range.
This patent outlines a thermocouple sensor with a sheath capable of extending the operating range of this class of sensor up to 1700° C. (3092° F.).